The present invention is directed to an explosive powder charge operated setting tool including a driving piston guided in the piston guide closed at its trailing end by a base part. The base part is penetrated by a centrally arranged channel opening to a powder charge chamber and with the trailing end face of the driving piston facing the base part having a depression.
Explosive charge operated setting tools, where the charge is usually ignited by percussion or a blow and the driving piston is accelerated by the high pressure generated due to the combustion of the powder charge, are especially used where a high volume of fastening elements are driven in a series arrangement. The explosive powder charges can be sleeveless cartridges formed of a powder stamping, or cartridges consisting of a metal housing, a plastic housing or a combination of the two with a conventional explosive powder filling the housing.
A specific power output of the setting tool is required depending upon the particular application. The output depends upon the pressure acting on the driving piston and this pressure is a function of the quantity of the powder used. Further, the pressure depends also on design features of the setting tool parts, such as gas tightness, swept volume, weight relationships and the like.
Experience has shown that the combustion of the explosive powder in the charge occurs extraordinarily quickly at the commencement of ignition. As a result, the portion of the powder located in the immediate vicinity of a detonator, usually located in the base region of the cartridge, is ignited immediately, so that the pressure generated in this region propels the adjoining powder portion in an unignited state. After the explosive powder charges are ignited in the usual manner by a priming arrangement acting upon the detonator from an end facing away from the setting direction, the unignited portion of the powder is propelled in the setting direction, that is, in the direction of the driving piston and the piston guide. Accordingly, the unburned portions of the powder propelled in the setting direction lead to fouling of the tool parts, particularly the piston guide and the driving piston, and result in functional or operating problems.
To eliminate these disadvantages it has been attempted to provide a space for receiving the unburned portion of the powder, so that it cannot penetrate into the guidance gaps between the driving piston and the piston guide and thus foul these parts. The trailing end face of the driving piston is available for forming such a space as disclosed in CH-PS 3 666 007.
In this known apparatus, as it is disclosed in the above-mentioned patent publication, the problems involving the fouling or contamination of the tool parts can be eliminated to a great extent, since the unburned portion of the powder reaches the centrally disposed depression in the end face of the driving piston. This depression has the particular effect that pressure is relieved, so that the unburned portion of the powder enters the guidance gap of the setting tool. This improvement, however, is counterbalanced by another disadvantage, that is, the pressure relief due to the depression results in a loss of output. Such loss of output depends on the volume of the depression, wherein if the depression volume is too small it would be less damaging to the loss of output, this, however, leads to the disadvantage that the pressure relief becomes too small, so that the unburned portion of the powder can penetrate into the guidance gap.